Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices include a plurality of optical modules including optical elements, such as lenses and mirrors etc., arranged in the light path of the optical system. Those optical elements, as a primary function of an exposure process, cooperate to transfer an image of a pattern formed on a mask, reticle or the like onto a substrate such as a wafer. The optical elements are usually combined in one or more functionally distinct optical element groups. These distinct optical element groups may be held by distinct optical exposure units. In particular with mainly refractive systems, such optical exposure units are often built from a stack of optical element modules holding one or more optical elements. These optical element modules usually include an external generally ring shaped support device supporting one or more optical element holders each, in turn, holding an optical element.
Due to the ongoing miniaturization of semiconductor devices there is a desire for enhanced resolution of the optical systems used for fabricating those semiconductor devices. This desire for enhanced resolution tends to push the desire for an increased numerical aperture (NA) and increased imaging accuracy of the optical system.
One approach to achieve enhanced resolution is to reduce the wavelength of the light used in the exposure process. In the recent years, approaches have been made to use light in the extreme ultraviolet (EUV) range using wavelengths ranging from 5 nm to 20 nm, typically about 13 nm. In this EUV range it is not possible to use common refractive optics any more. This is due to the fact that, in this EUV range, the materials commonly used for refractive optical elements show a degree of absorption that is too high for obtaining high quality exposure results on a commercially acceptable scale. Thus, in the EUV range, reflective systems including reflective elements such as mirrors or the like are used in the exposure process to transfer the image of the pattern formed on the mask onto the substrate, e.g. the wafer.
The transition to the use of high numerical aperture (e.g. NA>0.4 to 0.5) reflective systems in the EUV range leads to considerable challenges with respect to the design of the optical imaging arrangement.
One of the accuracy properties is the accuracy of the position of the image on the substrate, which is also referred to as the line of sight (LoS) accuracy. The line of sight accuracy typically scales to approximately the inverse of the numerical aperture. Hence, the line of sight accuracy is a factor of 1.4 smaller for an optical imaging arrangement with a numerical aperture NA=0.45 than that of an optical imaging arrangement with a numerical aperture of NA=0.33. Typically, the line of sight accuracy ranges below 0.5 nm for a numerical aperture of NA=0.45. If double patterning is also to be allowed for in the exposure process, then the accuracy would typically have to be reduced by a further factor of 1.4. Hence, in this case, the line of sight accuracy would range even below 0.3 nm.
Among others, the above leads to very strict desired properties with respect to the relative position between the components participating in the exposure process. Furthermore, to reliably obtain high-quality semiconductor devices it is not only desirable to provide an optical system showing a high degree of imaging accuracy. It is also desirable to maintain such a high degree of accuracy throughout the entire exposure process and over the lifetime of the system. As a consequence, the optical imaging arrangement components, i.e. the mask, the optical elements and the wafer, for example, cooperating in the exposure process is supported in a defined manner in order to maintain a predetermined spatial relationship between the optical imaging arrangement components to provide a high quality exposure process.
To maintain the predetermined spatial relationship throughout the entire exposure process, even under the influence of vibrations (introduced, among others, via the ground structure supporting the arrangement and/or via internal sources of vibration disturbances, such as accelerated masses, e.g. moving components, turbulent fluid streams, etc.) as well as under the influence of thermally induced position alterations, it is desirable to at least intermittently capture the spatial relationship between certain components of the optical imaging arrangement in an auxiliary process and to adjust the position of at least one of the components of the optical imaging arrangement as a function of the result of this auxiliary capturing process.
Typically, this auxiliary capturing process is done via auxiliary components of a metrology system using a central support structure for the optical projection system and the substrate system as a common reference in order to be able to readily synchronize motion of the actively adjusted parts of the imaging arrangement.
On the other hand, an increase in the numerical aperture, typically, leads to an increased size of the optical elements used, also referred to as the optical footprint of the optical elements. The increased optical footprint of the optical elements used has a negative impact on their dynamic properties and the control system used to achieve the above adjustments. Furthermore, the increased optical footprint typically leads to larger light ray incidence angles. However, at such increased light ray incidence angles transmissivity of the multi-layer coatings typically used for generating the reflective surface of the optical elements is drastically reduced, obviously leading to an undesired loss in light power and an increased heating of the optical elements due to absorption. As a consequence, even larger optical elements have to be used in order to enable such imaging at a commercially acceptable scale. These circumstances lead to optical imaging arrangements with comparatively large optical elements having an optical footprint of up to 1 m×1 m and which are arranged very close to each other with mutual distances ranging down to less than 60 mm.
Several problems result from this situation. First, irrespective of the so-called aspect ratio (i.e. the thickness to diameter ratio) of the optical element, a large optical element generally exhibits low resonant frequencies making it more susceptible to vibration disturbances typically experienced in the environment of such an optical imaging apparatus. While, for example, a mirror with an optical footprint of 150 mm (in diameter) and a thickness of 25 mm typically has resonant frequencies above 4000 Hz, a mirror with an optical footprint of 700 mm, typically, hardly reach resonant frequencies above 1500 Hz even at a thickness of 200 mm.
Furthermore, the increased thermal load on the optical elements used (due to light energy absorption) and the increased throughput desired for such systems involves increased cooling efforts via further auxiliary components such as coolers and, in particular, higher flow rates of the cooling fluids used. This increased cooling flow rate is prone to lead to an increase in the vibration disturbances introduced into the system due to cooling medium turbulences, in turn leading to reduced line-of-sight accuracy.
Hence, there is a desire to mechanically decouple far as possible the support of the optical elements used in the exposure process and the support of the auxiliary components of the metrology system and the cooling system from each other as well as from other internal and external vibration sources.
The increased size of the optical imaging apparatus and the multitude of auxiliary components lead to highly complex interleaved but mutually mechanically decoupled support structures. Such interleaved but decoupled support structures pose considerable problems during assembly of the optical imaging apparatus since the individual, ultimately mechanically decoupled components are held in a defined mutual position and orientation while, due to the interleaved arrangement, being hardly accessible for this purpose.